In the study of electronic materials and processes for fabricating such materials into electronic devices, a thin specimen is frequently required for analysis and for process validation. For instance, thin specimens are frequently used in the analysis of semiconductor structures by a transmission electron microscopy (TEM) method. TEM is one of the more popular methods used in analyzing the microscopic structures of semiconductor devices. The advantages achieved by a TEM method over that of a scanning electron microscopy (SEM) method are higher magnification and simpler specimen preparation since no staining is required, even though a more three dimensional image can be obtained by the SEM method.
In preparing thin specimens of semiconductor structures for a TEM investigation, various polishing and milling processes are involved so that specimens having thicknesses less than 1 .mu.m can be obtained. As device dimensions are continuously being reduced to the sub-half-micron level, the use of thin specimens for study by the TEM method becomes more important. In general, when a thin specimen is prepared for a TEM study, various mechanical polishing methods are first used to bring the dimension of the specimen down to its approximate dimension. A final sample preparation process is then accomplished by a method called ion milling. The ion milling method is frequently conducted by a focused ion beam (FIB) technique. In the FIB technique, focused ion beams are used to either locally deposit or remove materials. A cluster of ionized beam consists of an aggregate of 100 to 2,000 atoms. When the cluster impacts on the surface of a semiconductor structure, the cluster disintegrates into atoms which are then scattered over the surface and deposit to form a film. The technique can be used to deposit single crystal metals such as epitaxial aluminum on a silicon substrate or single crystal oxides such as silicon oxide on a silicon surface.
Focused ions beams have also been used in a wide range of applications for restructuring semiconductor circuits after they have been fabricated. For instance, the restructuring process includes mask repairing, micromachining, device cross-sectioning and fabrication of structures with dimensions less than 100 nm. Typical ion beams have a focused spot size of smaller than 100 nm produced by a high intensity source. Sources of such high intensity ions are either liquid metal ion sources or gas field ion sources. Both of these sources have a needle type form that relies on field ionization or evaporation to produce the ion beam. After the ion beam is produced, it is deflected in a high vacuum and directed to a desired area without requiring a masking step.
The focused ion beams can be used in the semiconductor processing industry as a cutting or attaching method when performing circuit repair, mask repair or micromachining processes. A cutting process can be performed by locally sputtering the material with a focused ion beam while the attaching process can be performed by a focused ion beam induced deposition process. For instance, a tungsten deposition process is typically used to connect metal lines by directing a focused beam of gallium ions to the region to be connected in an environment of hexacarbonyltungsten. In the process, the gallium ions cause the decomposition of hexacarbonyltungsten so that tungsten deposits locally at the desired location. The technique can also be used in a local rewiring process by etching a hole in the oxide and then locally depositing tungsten without shorting to any adjacent conductors.
In an ion beam milling process, when a material is selectively etched by a beam of ions such as Ga.sup.+ that is focused to a sub-micron diameter, the technique is often referred to as focused ion beam (FIB) etching or milling. FIB milling is a very useful technique for restructuring a pattern on a mask or an integrated circuit, and for diagnostic cross-sectioning of microstructures. In a typical FIB assisted etching process, a beam of ions such as Ga.sup.+ is incident on a surface to be etched and can be deflected to produce a desirable pattern. In the etch chamber, a gas such as Cl.sub.2 can be introduced to fill the chamber to about 30 m Torr, while the vacuum outside the chamber where the FIB is generated is normally maintained at approximately 10.sup.-7 Torr. In this sample system, the etch rates for both Si and GaAs have been increased. For instance, the etch rate of GaAs in a Cl.sub.2 environment by the FIB technique is about 10 times higher than the etch rate occurred in a chamber without the Cl.sub.2 gas. It is theorized that, similar to a reactive ion etching process, chemical reactions are induced or accelerated by the impact of a focused beam of energetic ions. The spatial resolution of the process is frequently limited by the spot size of the focused ion beam that can be produced.
While the technique of FIB micromachining has been used in preparing thin specimens of semiconductor devices, problem occurs when a focused ion beam is used to bombard a device surface that contains domains of different materials that have different densities. This occurs even when the angle of ion bombardment is very low, i.e., between 0.degree. and 5.degree., as long as there is a large difference between the densities of the two materials. One of such structures can be found in a semiconductor device which has a via or plug formed of a refractory metal in a dielectric material matrix formed of silicon oxide. Since the density of a refractory metal, i.e., tungsten, titanium, etc., is significantly higher than the density of a dielectric material, the sputtered ion beam used in an ion milling process removes the dielectric material at a higher rate than for the refractory metal.
It is known that the etch rates of different semiconductor materials is directly proportional to its densities, a lower density material such as silicon oxide is normally etched at a much higher etch rate than a higher density material such as tungsten. The great disparity in the etch rates leads to a significant processing difficulty in that before a refractory metal plug is micromachined to a desirable thickness, the surrounding silicon oxide material has already been completely etched away, or being etched to such a small thickness that it cracks or breaks. The refractory metal plug is therefore completely unsupported by the dielectric material that it was embedded in. The preparation of a refractory metal plug specimen that is thin enough for the electrons to penetrate through during a TEM examination is therefore extremely difficult.
It is therefore an object of the present invention to provide a method for preparing thin specimens consisting of domains of materials of different densities that does not have the drawbacks and shortcomings of the conventional preparation methods.
It is another object of the present invention to provide a method for preparing thin specimens consisting of domains of materials of different densities and consequently, of different etch rates.
It a further object of the present invention to provide a method for preparing thin film specimens consisting of domains of different materials by a focused ion beam milling technique.
It is another further object of the present invention to provide a method for preparing thin specimens consisting of domains of different materials by a FIB milling technique wherein support structures are first built to support a domain of material that has high density and low etch rate.
It is still another object of the present invention to provide a method for preparing thin specimens consisting of domains of different materials by a FIB milling technique by first forming two cavities in the domain of material that has low density and then depositing a high density material in the cavities.
It is yet another object of the present invention to provide a method for preparing thin specimens consisting of domains of different materials by a FIB milling technique wherein two support structures are first built juxtaposed to a domain of material that has high density such that the domain can be adequately supported during the milling process.
It is still another further object of the present invention to provide a method for preparing thin specimens consisting of domains of different materials by a FIB milling technique in which a domain of a tungsten plug is embedded in a domain of silicon oxide.
It is yet another further object of the present invention to provide a method for preparing thin specimens consisting of domains of different materials by a FIB milling technique wherein a platinum support structure is first built juxtaposed to a tungsten plug for supporting the plug during a subsequent milling process.